Circular patch microstrip antennas are well known in the art and have many advantages which make them particularly adapted for certain applications. In particular, a stacked microstrip patch antenna is relatively inexpensive and easily manufactured, rugged, readily conformed to surface mount to an irregular shape, has a broad reception pattern, and can be adapted to receive multiple frequencies through proper configuration of the patches.
One particular application includes utilizing a stacked microstrip patch antenna on an air frame for receiving signals transmitted by the Global Positioning System (GPS) satellites. In this application, the antenna must operate at dual frequencies and be physically small enough to be utilized in an array. Furthermore, the antenna should provide approximately hemispherical coverage and have its pattern roll-off sharply between 80.degree. and 90.degree. from broadside to reject signals from emitters on the horizon. Because of its conformability, the antenna is uniquely adapted for mounting to the host vehicle which could be double curved, and its characteristics provide a minimum impact on radar signature. The antenna must provide at least a 1.6% frequency bandwidth and circular polarization at both GPS frequencies. The antenna is ideal for use in a multi-element array for adaptive processing; a method of automatically steering nulls toward interfering signals. For this application, the antenna must provide at least 5% frequency bandwidth for good performance.
Some of the stacked microstrip antennas which are available in the prior art include the antenna disclosed in U.S. Pat. No. 4,070,676 which has square shaped microstrip patches stacked for dual frequency. However, based on the inventors' experience, this antenna does not exhibit the necessary frequency bandwidth for utilization as a GPS adaptive antenna. Still another microstrip patch antenna is disclosed at p. 255 of the 1984 IEEE Antennas and Propagation Digest which utilizes a triple frequency stacked microstrip element. However, once again the antenna bandwidth is not large enough to enable its use in a GPS adaptive antenna application. Still another stacked microstrip patch antenna is disclosed at p. 260 of the 1978 IEEE Antennas and Propagation Digest and this antenna has a pair of circular disks stacked one atop the other with a single feed extending through a hole in the lower disk and physically connected to the upper disk. However, as with the other antennas, this antenna does not exhibit the necessary frequency bandwidth to be utilized in a GPS adaptive antenna application.
The inventors herein have succeeded in developing an improved feed incorporating feed pins which are coupled to one of the patches for a dual frequency stacked circular microstrip patch antenna which increases the bandwidth including a wider frequency operating range within a prescribed VSWR, and a wider operating range for a prescribed antenna gain which permits its use with a GPS system, and especially with an adaptive nulling processor for interference rejection. The wider bandwidth permits the processor to develop deep nulls over a wide frequency range as is necessary for this system. The improved, wider bandwidth also minimizes the deleterious effects caused by manufacturing tolerances and environmental conditions which would otherwise shift a narrower band antenna out of the desired frequency range.
The dual frequency microstrip patch antenna includes two circular microstrip patches stacked concentrically, one over the other, with each patch resonating at a different frequency. In this improved design, only the upper patch has a direct connection with the feed network by way of two vertical feed through pins while the lower patch receives its excitation by capacitive coupling. The inventors herein have discovered that the feed through hole size and shape directly affect the frequency bandwidth of each patch while operating at their separate frequencies typical for a GPS antenna. With many of these holes, considerable bandwidth improvements were realized over using a standard, prior art, round feed through holes. In analyzing the results, four separable characteristics of the holes were identified for purposes of interpreting the resulting increased bandwidths. A hole was considered "elongated" if its length along the patch radius was longer than the circumferential length. A hole was considered "tapered" if its width narrowed more as the hole approached the patch outer edge compared to the opposite direction. The hole was considered "rounded" if the end toward the patch outer edge had a radius instead of converging to a sharp point. Lastly, the hole shape was considered "smooth" if there were no sharp corners anywhere over the hole circumference. In the final analysis, it was apparent that all four characteristics were important for an increased bandwidth. As explained in greater detail below, elongated, rounded, and smooth characteristics were common to the two shapes giving the best lower frequency bandwidth. On the other hand, elongated and tapered characteristics were common to the three hole shapes giving the widest upper frequency bandwidth. The one hole shape which included all four characteristics appeared to be the best compromise in that it provided the largest bandwidth at the lower frequency and the third largest bandwidth at the upper frequency.
The antenna of the present invention is comprised of eight boards, some of which have a copper layering on one or both sides thereof, and others of which have no copper and are used as spacers. Furthermore, the boards themselves may be of varying thicknesses although in the preferred embodiment the top five boards are substantially the same thickness and the bottom three boards are smaller than the top five boards. From top to bottom, the eight boards can be generally described as follows:
Board No. 1 has an upper layer of copper configured in a circle to form the upper patch.
Board No. 2 is a layer of dielectric with no copper on either side.
Board No. 3 has an upper layer of copper to form the lower patch and has a pair of feed through holes which can be shaped in accordance with one of the several embodiments disclosed herein to accommodate insertion of feed pins.
Board No. 4 is a layer of dielectric with no copper on either side.
Board No. 5 is a layer of dielectric with no copper on either side.
Board No. 6 is a dielectric with a layer of copper along its upper surface with a pair of circles cut out on its upper side for the feed pins to pass through.
Board No. 7 is a dielectric of greatly reduced thickness having a copper trace on the upper and lower sides forming the backward wave coupler.
Board No. 8 is a dielectric of reduced thickness with copper layering on the bottom except for two circular patches to accommodate termination and feed connections for the backward wave coupler.
In addition to the modal pin which interconnects both the upper and lower patches to the two ground planes, a number of cavity pins extend between the ground planes surrounding the two feed connections. Also, two pins connect the upper patch to the backward wave coupler.
By bonding these boards together, a rigid structure is formed which can be conformed to fit the surface on which the antenna is to be mounted and yet provide a low profile. Furthermore, with the feed through hole design of the present invention, an increased bandwidth is achieved which enables the antenna to be used in a GPS system.
While the principal advantages and features of the present invention have been briefly described, a more complete understanding of the invention may be obtained by referring to the drawings and the Detailed Description of the Preferred Embodiment which follows.